Lead in tin-lead solders has been proved to be toxic. People worry that high-lead solder in tin-lead solders will be harmful to the environment and the human. EU is very active in legislation to prohibit the use of lead in the electronics industry. In June 2000, European Union issued two regulations of Waste Electrical and Electronic Equipment Directive (WEEE) and Restriction of Hazardous Substances (RoHS). In lead-free solders, 95.4Sn/3.1Ag/1.5Cu has been widely considered to be the most promising lead-free solder. However, such solder has a melting point of 217° C., which is higher than that of the lowest tin-lead solder (183° C.) by 30° C.
In order to make the lead-free solder obtain sufficient wetting during processing, the processing temperature needs to be 30-40° C. higher than the melting point during the electronic assembly process. The increase of the temperature reduces the integration, reliability and functionality of the printed circuit board, components and its accessory parts.
Conductive adhesive is mainly composed of organic polymer matrix and metal filler, wherein the conductive filler provides conductive property, and the polymer matrix provides physical and mechanical properties. As compared with metal solder, conductive adhesive has many advantages, such as environment-friendly (no lead and reflux cleaning agent), mild processing conditions, less processing steps (lowering processing costs), especially fine pitch connection capability formed by using small particles. Although conductive adhesives have many advantages, they are still in infancy as compared to mature tin-lead solders, and there are many shortcomings and challenges that need to be addressed, such as lower conductivity and thermal conductivity as compared to the materials connected with solder, conductive fatigue in reliability testing, limited current carrying capacity, metal migration fatigue and poor impact strength in high pressure and reliability testing.
The thermal conductivity of graphene is as high as 5300 W/m·K, which is higher than those of carbon nanotubes and diamonds. The electron mobility thereof at room temperature is more than 15000 cm2/V·s, which is higher than that of carbon nanotubes or silicon crystals. The resistivity thereof is only about 10−6 Ω·cm, which is lower than that of copper or silver and is the material having the world's smallest resistivity. Because of its very low resistivity, the electron migration thereof is extremely fast and it has special electronic properties. As compared with the expensive fullerene and carbon nanotubes, graphene is inexpensive and the raw materials are easy to obtain. Thus graphene is expected to be the high-quality filler of the polymer nanocomposites. At present, it has been studied as a modifier of precious metals such as metallic silver, so as to improve the electrical conductivity of the conductive adhesive after the addition.
The conventional conductive adhesive achieves a bonded conductive interconnect to the semiconductor by adding a large amount of precious metals such as metallic silver or graphene surface modified metallic silver to a resin matrix composed of an epoxy resin, a curing agent, an accelerator and adjuvant(s), and then by heating or light-curing. The current studies focus more on how to increase the amount of conductive filler or to modify the surface of the conductive filler so as to improve the electrical conductivity. But the study on the resin system of the conductive adhesive is very less, especially curing agent. Curing agent has great effect on the use characteristics of the conductive adhesive, and the performance of the thermosetting polymer formed by curing and having a three-dimensional network structure.
CN102010685A uses one or more curing agents selected from dicyandiamide, modified imidazole and its derivatives and modified amine curing agents. Said curing agents are used to cure bisphenol A epoxy resin or bisphenol F epoxy resin, and the resultant cured product forms a thermosetting polymer having a three-dimensional network structure. Such thermosetting polymer has the shortcomings, such as being difficult to melt, being difficult to dissolve in organic solvents, resulting in more processing steps, high cost and complex operations for recovering precious metals (e.g. flake silver powder, nano silver particles) and graphene in conductive adhesive. In addition, CN102010685A discloses using conventional dicyandiamide, or imidazole curing agents. Since these conventional curing agents have worse thermal resistance (dicyandiamide), or have too much rigidity (aromatic amines), insufficient shear strength, they are difficult to balance among heat resistance, toughness and rigidity. CN102925100A also discloses using conventional curing agents and also has the aforesaid problems.